ATLANTA — A salmonella outbreak that has sickened 810 Americans may still be making people ill, and it may not be linked to tomatoes after all, federal health authorities said Friday.

Tomatoes still have the strongest association with the outbreak, said Patricia Griffin of the Centers for Disease Control and Prevention. But investigators are now looking at other ingredients that may be in foods eaten by salmonella victims, which include such popular Tex-Mex dishes as guacamole and fresh salsa.

They're also investigating whether tomatoes from growing regions cleared in the outbreak could be picking up the bacteria in a packing house or distribution warehouse and causing infections in people who think they're buying safe tomatoes.

"Whatever the produce item is that's causing the illness is possibly still out there making people sick," said Griffin, chief of the CDC's Enteric Diseases Epidemiology Branch.

For now, the U.S. Food and Drug Administration has not changed its advice. The FDA says that cherry and grape tomatoes and tomatoes on the vine are safe to eat, and that it's also OK to eat round red tomatoes and Roma, or plum tomatoes, from approved growing regions.

The FDA cleared areas that weren't producing tomatoes when the outbreak began April 10. Tomatoes from some of those regions are going through the same packing houses and distribution centers that the FDA is checking as part of its attempts to trace back the contaminated tomatoes, David Acheson, the agency's associate commissioner for foods, said in conference call with reporters.

At this time, the FDA has no indication that cleared growing regions have any connection to the outbreak, Acheson said.

That could change.

"We need to re-examine all parts of this system and make sure the consumer message is still solid," Acheson said.

In the past week, investigators have taken 1,700 samples of water, tomatoes and soil from farms, packing houses and warehouses in Mexico, Florida and other parts of the United States, looking for the Salmonella Saintpaul strain that is causing the illnesses. So far, no bacteria have been found.

"If it's not tomatoes, we've gone through a lot of agony," said Charles Hall, executive director of the Georgia Fruit and Vegetable Growers Association.

Warnings about the outbreak have scared some buyers away from any tomatoes, and Georgia farmers are seeing prices drop as they approach their peak production season.

"It's rather frustrating," Hall said. "I know it is from the consumer side, too, if we can't determine exactly what's causing this continuation of the outbreak."

The outbreak has stretched to 36 states and the District of Columbia, with more than half the cases in Texas and New Mexico.

Illnesses started April 10 and were occurring as recently as June 15, just 13 days ago. On average, it takes 16 days to get laboratory confirmation of a food-borne illness, Griffin said. With a heavy concentration of cases in late May and more showing up in early June, the outbreak doesn't appear to be over, she said.

It's possible that because of a long growing season, contaminated tomatoes could still be coming from a farm, Acheson said. Contamination could also be occurring at other steps along the distribution chain.

A tomato industry practice of grouping vegetables by size and ripeness, not by the farm that produced them, has complicated the investigation, Acheson said.

He cited an industry estimate that as many as 90 percent of tomatoes are repacked in that manner. Tomatoes from the United States and other countries may be mixed in the same box during repacking, he said. And tomatoes grown in the United States may be sent to Mexico for sorting, then shipped back to the United States and tagged as a product of this country.

The practice "makes tracing extremely difficult," Acheson said.

Investigators may never find the source of the contamination, not unusual for food-borne illnesses linked to tomatoes, he said.

There have been 13 salmonella outbreaks tied to tomatoes in the past decade. The most recent is also the largest. The second-largest, in 2002, sickened 510.

Health authorities estimate there may be as many as 30 unreported illnesses for every laboratory-confirmed case.